Continuous hydrogen gas evolution by self-flocculated cells of Enterobacter aerogenes, a natural isolate HU-101 and its mutant AY-2, was performed in a packed-bed reactor under glucose-limiting conditions in a minimal medium. The flocs that formed during the continuous culture were retained even when the dilution rate was increased to 0.9 h(-1). The H-2 production rate increased linearly with increases in the dilution rate up to 0.67 h(-1), giving maximum H-2 production rates of 31 and 58 mmol l(-1) h(-1) in HU-101 and AY-2 respectively. at a dilution rate of more than 0.67 h(-1). The molar H-2 yield from glucose in AY-2 was maintained at about 1.1 at dilution rates between 0.08 h(-1) and 0.67 h(-1), but it decreased rapidly at dilution rates more than 0.8 h(-1).